They might be fab constrained on the A8 chip. With A8X spanning 3B trannies, that's a fair chunk of silicon, and the volumes Apple demand... Maybe they felt the mini was good enough as-is. It's not their flagship product anyhow.
Apple is still selling the A7-powered iPad Air at the same price as the iPad Mini 3 [Retina], so they probably didn't want to create a scenario where people would choose one over the other based on application processor.